1. Field of the Invention
The present invention relates to the preparation and processing of mineral-filled elastomers. More particularly, the present invention relates to a process for the preparation of elastomer compositions comprising inorganic fillers, latent mercaptosilane coupling agents, mercaptosilane coupling agents, latent-mercaptosilane activators, and organic polymers containing carbonxe2x80x94carbon double bonds.
2. Description of Related Art
In the preparation of mineral-filled elastomer compositions, it is known to use as the coupling agent a polysulfide silane in which two alkoxysilyl groups are bound, each to one end of a chain of sulfur atoms. These coupling agents function by chemically bonding silica or other mineral fillers to the polymer when used in rubber applications in a relatively simple and straightforward manner. Coupling is accomplished by chemical bond formation between the silane sulfur and the polymer and by hydrolysis of the silane alkoxy groups, followed by condensation with silica hydroxyl groups.
WO 98/53004 discloses that polysulfide silanes can be used as part of a mineral-treatment process and that these fillers can then be used to prepare rubber masterbatches that are, or can subsequently be, converted to mineral-filled rubber.
Canadian 2231302 A1 also describes the preparation of mineral-filled elastomers containing polysulfide silanes.
U.S. Pat. No. 3,957,718 discloses silica containing mixtures for the improvement of the adhesion of vulcanizable mixtures of natural and/or synthetic rubber to reinforcing fillers or supports of textile and/or metallic fabrics after the vulcanization are prepared that substantially consist of (A) active synthetically produced silica or silicates having a specific surface area according to the BET procedure of about 50 to 500 m2/g and an average primary particle size of from about 5 to 100 and at least one of (B) phenolic resin or aminoplast forming components, namely on the one hand phenols and/or amines and on the other hand aldehydes or aldehyde donors, and/or at least (C) one organosilane which can be a bisalkoxysilylalkyl-oligosulfide of a specified formula.
U.S. Pat. No. 4,184,998 discloses the bonding of rubber or other plastics to mineral reinforcements, such as silica pigment. The bonding is brought about by a mercapto alkyl silane without production of foul odors from the mercapto compound by preliminary reversible coupling of the mercapto compound with a material capable of addition reaction, such as toluene diisocyanate. However, the reversible decomposition of the addition product generates toxic isocyanates and poses a health risk to people.
U.S. Pat. No. 4,519,430 discloses a pneumatic radial tire with a tread having from one part by weight of hydrated amorphous fine-particle silica per three parts of carbon black tread reinforcing filler, to about three parts by weight of the silica to one part of the carbon black. The amount of silica present is preferably in the range from about 18 parts to about 50 parts by weight per 100 parts of natural or synthetic rubber. When such a tread is compounded with a mercaptosilane in which the mercapto group is reversibly blocked, the tread is said to provide reduced rolling resistance without loss of traction.
U.S. Pat. No. 4,820,751 discloses a rubber composition for tires said to have improved heat build-up and high temperature reinforcibility that comprises specified amounts of particular surface-treated carbon black, silica and particular silane coupling agent based on 100 parts by weight of the rubber ingredient. The preparation of compositions with these silanes, however, is not desirable because the xcex1-xcex2-unsaturated carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage.
U.S. Pat. No. 5,227,425 discloses a sulfur-vulcanizable rubber composition obtained by thermomechanical working of a conjugated diene compound and an aromatic vinyl compound prepared by solution polymerization in a hydrocarbon solvent having a total content of aromatic vinyl compound of between 5% and 50% and a glass transition temperature (Tg) of between 0xc2x0 C. and xe2x88x9280xc2x0 C. with 30 to 150 parts by weight per 100 parts by weight of elastomer of a silica having a BET surface area of between 100 and 250 m2/g, a CTAB surface area of between 100 and 250 m2/g, an oil absorption measured in DBP of between 150 and 250 mL/100 g, and an average projected area of the aggregates greater than 8500 nm2 before use and between 7000 and 8400 nm2 after thermomechanical mixing as well as the additives conventionally employed, with the exception of the sulfur vulcanization system, comprising at least one heat step reaching a temperature of between 130xc2x0 C. and 180xc2x0 C. for a suitable period of time of between 10 seconds and 20 minutes that is a function of the temperature selected in order to carry out the mechanical work and of the nature and volume of the components subjected to the mechanical work, followed by a finishing step consisting of the incorporating of the vulcanization system by mechanical work at a temperature below the vulcanization temperature.
U.S. Pat. No. 5,650,457 discloses rubber mixtures containing at least one rubber, a vulcanizing agent, a filler, optionally other rubber additives and at least one reinforcing additive of a specified formula, which are used for the production of vulcanized rubbers, from which in particular tires can be produced said to have a low rolling resistance combined with a good wet skid resistance and a high abrasion resistance.
U.S. Pat. No. 5,679,728 discloses a rubber composition containing a cross-linkable rubber component and a carbon black having silica adhered to the surface thereof and a silica surface-treated carbon black suitable for use in tire tread where the surface-treated carbon black has a nitrogen specific surface area of 20 to 300 m2/g, a ratio of the adsorption amount of iodine and the adsorption amount of iodine after treatment by hydrogen fluoride of at least 0.30 but less than 1.0, and a ratio of the adsorption amount of iodine after extraction from a rubber compound and the value of the same after treatment by hydrogen fluoride of at least 0.30 but less than 1.0.
U.S. Pat. No. 6,127,468 discloses the use of blocked mercaptosilanes in filled polymers, wherein the hydrogen atom of the mercaptan functionality has been substituted. The blocked mercaptosilanes described are unique in that they allow the mixing of fillers with organic polymers to proceed with surpressed reactivity toward coupling to the polymer. The coupling reactions of these blocked mercaptosilane compounds are triggered subsequently by addition of an appropriate deblocking agent, which preferably is tetramethylthiuram monosulfide.
JP 63270751 A2 discloses the preparation of rubber compositions for use in tire treads, using silane coupling agents represented by the general formula, CH2xe2x95x90C(CH3)C(xe2x95x90O)S(CH2)1-6Si(OCH3)3. The preparation of compositions with these silanes, however, is not desirable because the unsaturation adjacent to the carbonyl group of the thioester has the undesirable potential to polymerize during the compounding process or during storage.
AU-A-10082/97 discloses the preparation of rubber compounds using silanes of the structure represented by R1nX3-nSixe2x80x94(Alk)m(Ar)pxe2x80x94S(Cxe2x95x90O)xe2x80x94R (Formula 1) where R1 is phenyl or alkyl; X is halogen, alkoxy, cycloalkoxy, acyloxy, or OH; Alk is alkyl; Ar is aryl; R is alkyl, alkenyl, or aryl; n is 0 to 2; and m and p are each 0 or 1, but not both zero. Compositions of the structure of Formula 1 must be used in conjunction with functionalized siloxanes.
U.S. Pat. No. 3,922,436 discloses carboxylic acid functional silances that are used as coupling agents in glass fiber reinforced elastomers. The silanes are formed by the reaction of an epoxy, a mercapto or an amino silane with a carboxylic acid or anhydride.
Other known means for the preparation of mineral-filled elastomers make use of sulfur-containing silane coupling agents that contain the mercapto (xe2x80x94Sxe2x80x94H) or the thiocarbonyl (xe2x80x94Cxe2x95x90S) group instead of the more commonly described polysulfide (xe2x80x94Sxe2x80x94) group.
Mercaptosilanes have offered superior coupling at substantially reduced loadings. However, their high chemical reactivity with organic polymers leads to unacceptably high viscosities during elevated temperature processing and premature curing (scorch). Their undesirability is aggravated by their odor and perhaps worse, the odor of their impurities. As a result, the polysulfide silane coupling agent and at times, other, less reactive, coupling agents are used in the currently practiced art and donate sulfur during the nonproductive mixes, which causes premature cross-linking and increases in viscosity. The polysulfides do not couple as well as the mercaptosilanes and require much higher loadings. The less reactive coupling agents, such as the thiocarbonyl functional silanes, give inferior performance properties when compared to the mercaptosilanes because the organofunctional group has not bonded completely with the organic rubber during the mixing and curing processes. Therefore, there exists a need for rubber compositions that do not undergo premature scorch at elevated temperature processing and maintain the performance of mercaptosilane-containing rubber compositions.
All references cited herein are incorporated by reference herein in their entirety.
In a first aspect, the present invention is directed to the preparation and processing of mineral-filled elastomers. In another aspect, the present invention is directed to additives that enhance the performance of latent mercaptosilanes. In still another aspect, the present invention is directed to additives that enhance the performance of the latent mercaptosilanes that are intended to be used with the mineral-filled elastomers.
More particularly, the present invention is directed to a composition of matter comprising:
A) at least one latent mercaptosilane coupling agent; and
B) at least one additional mercaptan.
In a preferred embodiment, the present invention is directed to a composition of matter comprising:
A) at least one sulfur vulcanizable rubber;
B) at least one latent mercaptosilane coupling agent;
C) at least one latent-mercaptosilane activator;
D) at least one filler; and, optionally,
E) at least one additional mercaptan, (referred to hereinafter as xe2x80x9cadditional mercaptanxe2x80x9d).
In another aspect, the present invention is directed to a process for the preparation of a rubber composition consisting essentially of the steps of:
A) thermomechanically mixing in a first pass:
1) at least one sulfur vulcanizable rubber;
2) at least one latent mercaptosilane coupling agent; and
3) at least one filler; and, optionally,
4) at least one additional mercaptan;
B) bringing the temperature of the first pass mix from about 140xc2x0 C. to about 200xc2x0 C. over a period of 1 to 20 minutes, preferably 150xc2x0 C. to 180xc2x0 C. over a period of about 2 to 10 minutes;
thereby forming a nonproductive first mixture;
C) thermomechanically mixing the nonproductive first mixture in a second pass, optionally in the presence of at least one curing agent, at a temperature of from about 25 to about 100xc2x0 C. for a period of from about 1 to about 30 minutes; thereby forming a productive second mixture;
D) adding at least one latent-mercalptosilane activator in step A), step C), or both step A) and step C);
E) shaping the second mixture; and then
F) curing (cross-linking) the second mixture at a temperature between about 130xc2x0 C. to about 200xc2x0 C. for a period of from about 5 to about 60 minutes.
Preferably, in the above-described process, at least one latent-mercaptosilane activator is added in step A). Optionally, the filler in step A) can be pretreated with at least a portion of the latent-mercaptosilane coupling agent.
In yet another aspect, the present invention is directed to a tire comprising a cured composition of matter comprising:
A) at least one sulfur vulcanizable rubber;
B) at least one latent mercaptosilane coupling agent;
C) at least one latent-mercaptosilane activator;
D) at least one filler; and, optionally,
E) at least one additional mercaptan.
In still another aspect, the present invention is directed to a tire tread comprising a cured composition of matter comprising:
A) at least one sulfur vulcanizable rubber;
B) at least one latent mercaptosilane coupling agent;
C) at least one latent-mercaptosilane activator;
D) at least one filler; and, optionally,
E) at least one additional mercaptan.
Where, in the above described aspects of the present invention, at least one additional mercaptan is present, it is preferably a mercaptosilane.
The present invention offers improvements in the preparation of elastomer compositions containing mineral fillers and silane coupling agents. These improvements are made possible by the choice of a coupling agent that extends the rigor with which the rubber can be processed without the detrimental effects that would accompany rubber processed using the silane coupling agents known heretofore. The improvements are also realized by proper modification of rubber compounding procedures and by the addition of promoters, hereinafter referred to as latent-mercaptosilane activators, which are chemical compounds that act in concert with latent mercaptosilanes used in the elastomer compositions of the present invention. These improvements include increased efficiency and the elimination of a mixing step in preparing the rubber compound. Whereas two mixing cycles, commonly called nonproductive mixes, in an internal mixer are typically required with the existing technology, and a third mixing cycle in either an internal mixer or a roll mill to add the curatives, the present invention permits the elimination of one of the mixing cycles. Only a single nonproductive mixing cycle in an internal mixer is required with a second productive mixing cycle in either an internal mixer or a roll mill. In addition to a change in the mix procedure and to accommodate this change, the present invention includes the use of more rigorous mixing conditions, such as higher temperatures, higher mixer-loading levels, and the use of activators at high temperatures prior to the final mixing stage.
The present invention relates to a process for the preparation of elastomer compositions comprising several specific types of ingredients, primarily including organic polymers containing carbonxe2x80x94carbon double bonds and fillers. The fillers are mineral fillers or combinations of mineral and carbon black fillers. Also included among the ingredients are precursor silanes that have the potential to form mercaptosilanes during the process of preparing rubber compositions, but which are not necessarily required to function via the formation of mercaptosilanes. Molecules of these precursor silanes, referred to herein as latent mercaptosilanes, have a blocking group in place of a hydrogen atom. The latent mercaptosilanes have the potential to become de-blocked by replacement of the blocking groups with a hydrogen atom during the rubber compounding and curing procedures to yield a more reactive mercaptosilane, defined herein as a silane whose molecular structure contains at least one mercapto (xe2x80x94SH) group bound to carbon and at least one silicon atom bound to carbon. Further, the ingredients of the elastomer compositions of the present invention may include one or more additional mercaptans, preferably mercaptosilanes. The ingredients also include latent-mercaptosilane activators, defined herein as chemical compositions that function and/or are intended to function in a manner so as to initiate, accelerate, and/or enhance the activity of the latent mercaptosilane in the application described herein. The latent-mercaptosilane activators and the latent mercaptosilane coupling agents junction together to permit the accelerated processing of the elastomer with fewer steps and under conditions more rigorous than those employed heretofore. The ingredients may also include other components known to those skilled in the art.
The latent mercaptosilanes of the present invention comprise at least one compound having the general structure depicted by Formulae 1 and 2:
{{(ROC(xe2x95x90O)xe2x80x94)p(Gxe2x80x94)j}kYxe2x80x94Sxe2x80x94}rG(xe2x80x94SiX3)sxe2x80x83xe2x80x83(1) 
and 
{(X3Sixe2x80x94)qG}a{Y{xe2x80x94Sxe2x80x94Gxe2x80x94SiX3}b}cxe2x80x83xe2x80x83(2) 
R is a moiety comprising from 1 to 18 carbon atoms, which, in each occurrence is independently selected from the group consisting of hydrogen; straight, cyclic, or branched alkyl that may or may not contain unsaturation; alkenyl groups; aryl groups; arenyl groups; and aralkyl groups. Preferably, R is hydrogen, aryl, or C1 to C4 alkyl.
Each occurrence of G is independently a fragment, obtained by substitution of a quantity of hydrogen atoms given by p, of any hydrocarbon or oxa-, aza-, and/or cyano-substituted hydrocarbon, G1, having from 1 to 18 carbon atoms. G1 includes, but is not limited to, 1) branched, straight-chain, cyclic, and/or polycyclic aliphatic hydrocarbons, optionally containing:
a. ether functionality via oxygen atoms each of which is bound to two separate carbon atoms, or
b. tertiary amine functionality via nitrogen atoms, each of which is bound to three separate carbon atoms, and/or
c. cyano (CN) groups;
2) aromatic hydrocarbons; and
3) arenes derived by substitution of the aforementioned aromatics with branched or straight chain alkyl, alkenyl, alkynyl, aryl, and/or aralkyl groups;
with the proviso that if G is univalent (i.e., if p=0), G can be a hydrogen atom.
Each occurrence of X is a moiety independently selected from the group consisting of ROxe2x80x94, RC(xe2x95x90O)Oxe2x80x94, R2Cxe2x95x90NOxe2x80x94, R2NOxe2x80x94R2Nxe2x80x94, xe2x80x94R, and xe2x80x94(OSiR2)t(OSiR3);
wherein each R and G is as described above and at least one X is not xe2x80x94R.
Y is a polyvalent species (Q)zA(xe2x95x90E).
Each occurrence of Q is independently selected from the group consisting of oxygen, sulfur, and (xe2x80x94NRxe2x80x94).
Each occurrence of A is independently selected from the group consisting of carbon, sulfur, phosphorus, and sulfonyl.
Each occurrence of E is independently selected from the group consisting of oxygen, sulfur, and NR.
The following variables are integers, each occurrence of which is independent:
a is 0 to 7;
b is 1 to 3;
c is 1 to 6, preferably 1 to 4;
j is 0 to 1, but is 0 only if p is 1;
k is 1 to 2;
p is 0 to 5;
q is 0 to 6;
r is 1 to 3;
s is 1 to 3;
t is 0 to 5; and
z is 0 to 2;
provided that:
(1) if j is 0, then p is 1;
(2) if A is carbon, sulfur, or sulfonyl, then:
(i) at least one of a and c is equal to 1;
(ii) a+b=2; and
(iii) k=1;
(3) if A is phosphorus, then
(i) the value of a does not exceed 2c; and
(ii) k=2.
Preferably, Y is selected from the group consisting of xe2x80x94C(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90NR)xe2x80x94; xe2x80x94SC(xe2x95x90O)xe2x80x94; (xe2x80x94NR)C(xe2x95x90O)xe2x80x94; (xe2x80x94NR)C(xe2x95x90S)xe2x80x94; xe2x80x94OC(xe2x95x90O)xe2x80x94; xe2x80x94OC(xe2x95x90S)xe2x80x94; xe2x80x94C(xe2x95x90O)xe2x80x94; xe2x80x94SC(xe2x95x90S)xe2x80x94; xe2x80x94C(xe2x95x90S)xe2x80x94; xe2x80x94S(xe2x95x90O)xe2x80x94; xe2x80x94S(xe2x95x90O)2xe2x80x94; xe2x80x94OS(xe2x95x90O)2xe2x80x94; (xe2x80x94NR)S(xe2x95x90O)2xe2x80x94; xe2x80x94SS(xe2x95x90O)xe2x80x94; xe2x80x94OS(xe2x95x90O)xe2x80x94; (xe2x80x94NR)S(xe2x95x90O)xe2x80x94; xe2x80x94SS(xe2x95x90O)2xe2x80x94; (xe2x80x94S)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90O)xe2x80x94; xe2x80x94P(xe2x95x90O)(xe2x88x92)2; (xe2x80x94S)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94S)P(xe2x95x90S)xe2x80x94; xe2x80x94P(xe2x95x90S)(xe2x88x92)2; (xe2x80x94NR)2P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90O)xe2x80x94; (xe2x80x94O)2P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90O)xe2x80x94; xe2x80x94(xe2x80x94NR)P(xe2x95x90O)xe2x80x94; (xe2x80x94NR)2P(xe2x95x90S)xe2x80x94; (xe2x80x94NR)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94NR)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)(xe2x80x94S)P(xe2x95x90S)xe2x80x94; (xe2x80x94O)2P(xe2x95x90S)xe2x80x94; xe2x80x94(xe2x80x94O)P(xe2x95x90S)xe2x80x94; and xe2x80x94(xe2x80x94NR)P(xe2x95x90S)xe2x80x94.
More preferably, Y is selected from the group consisting of xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94SC(xe2x95x90O)xe2x80x94, xe2x80x94S(xe2x95x90O)xe2x80x94, xe2x80x94OS(xe2x95x90O)xe2x80x94, xe2x80x94(xe2x80x94S)P(xe2x95x90O)xe2x80x94, xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94C(xe2x95x90S)xe2x80x94 and xe2x80x94P(xe2x80x94O)(xe2x88x92)2;
each wherein the atom (A) attached to the unsaturated heteroatom (E) is attached to the sulfur, which in turn is linked via a group G to the silicon atom.
Representative examples of the functional groups (xe2x80x94YSxe2x80x94) present in the latent mercaptosilanes of the present invention include thiocarboxylate ester, xe2x80x94C(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiocarboxylate ester silanexe2x80x9d); dithiocarboxylate, xe2x80x94Oxe2x80x94C(xe2x95x90S)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cdithiocarboxylate ester silanexe2x80x9d); thiocarbonate ester, xe2x80x94Oxe2x80x94C(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiocarbonate ester silanexe2x80x9d); dithiocarbonate ester, xe2x80x94Sxe2x80x94C(xe2x95x90O)xe2x80x94Sxe2x80x94 and xe2x80x94Oxe2x80x94C(xe2x95x90S)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cdithiocarbonate ester silanexe2x80x9d); trithiocarbonate ester, xe2x80x94SC(xe2x95x90S)Sxe2x80x94(any silane with this functional group is herein referred to as a xe2x80x9ctrithiocarbonate ester silanexe2x80x9d); dithiocarbamate ester, xe2x80x94Nxe2x80x94C(xe2x95x90S)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cdithiocarbamate ester silanexe2x80x9d); thiosulfonate ester, xe2x80x94S(xe2x95x90O)2xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfonate ester silanexe2x80x9d); thiosulfate ester, xe2x80x94Oxe2x80x94S(xe2x95x90O)2xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfate ester silanexe2x80x9d); thiosulfamate ester, (xe2x80x94Nxe2x80x94)S(xe2x95x90O)2xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfamate ester silanexe2x80x9d); thiosulfinate ester, Cxe2x80x94S(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfinate ester silanexe2x80x9d); thiosulfite ester, xe2x80x940xe2x80x94S(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfite ester silanexe2x80x9d); thiosulfimate ester, (xe2x80x94Nxe2x80x94)S(xe2x95x90O)xe2x80x94Sxe2x80x94 (any silane with this functional group is herein referred to as a xe2x80x9cthiosulfimate ester silanexe2x80x9d); thiophosphate ester, P(xe2x95x90O)(Oxe2x80x94)2(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cthiophosphate ester silanexe2x80x9d); dithiophosphate ester, P(xe2x95x90O)(Oxe2x80x94)(Sxe2x80x94)2 and P(xe2x95x90S)(Oxe2x80x94)2(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cdithiophosphate ester silanexe2x80x9d); trithiophosphate ester, P(xe2x95x90O)(Sxe2x80x94)3 and P(xe2x95x90S)(Oxe2x80x94)(Sxe2x80x94)2 (any silane with this functional group is herein referred to as a xe2x80x9ctrithiophosphate ester silanexe2x80x9d); tetrathiophosphate ester P(xe2x95x90S)(Sxe2x80x94)3 (any silane with this functional group is herein referred to as a xe2x80x9ctetrathiophosphate ester silanexe2x80x9d); thiophosphamate ester, xe2x80x94P(xe2x95x90O)(xe2x80x94Nxe2x80x94)(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cthiophosphamate ester silanexe2x80x9d); dithiophosphamate ester, xe2x80x94P(xe2x95x90S)(xe2x80x94Nxe2x80x94)(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cdithiophosphamate ester silanexe2x80x9d); thiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90O)(Oxe2x80x94)(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cthiophosphoramidate ester silanexe2x80x9d); dithiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90O)(Sxe2x80x94)2 and (xe2x80x94Nxe2x80x94)P(xe2x95x90S)(Oxe2x80x94)(Sxe2x80x94) (any silane with this functional group is herein referred to as a xe2x80x9cdithiophosphoramidate ester silanexe2x80x9d); and trithiophosphoramidate ester, (xe2x80x94Nxe2x80x94)P(xe2x95x90S)(Sxe2x80x94)2 (any silane with this functional group is herein referred to as a xe2x80x9ctrithiophosphoramidate ester silanexe2x80x9d).
A preferred latent mercaptosilane is one wherein Y is xe2x80x94C(xe2x95x90O)xe2x80x94 and wherein j is 1, k is 1, p is zero, and G has a primary carbon atom attached to the carbonyl and is a C2-C12 alkyl, more preferably a C6-C8 alkyl. These latent mercaptosilanes can be represented by Formula 3, which is a more specific version of the structure depicted by Formula 1 above:
(GYSxe2x80x94)rG(xe2x80x94SiX3)sxe2x80x83xe2x80x83(3) 
Another preferred latent mercaptosilane has a structure of the form X3SiGSC(xe2x95x90O)GC(xe2x95x90O)SGSiX3 wherein G is a divalent hydrocarbon.
Representative examples of G include xe2x80x94(CH2)nxe2x80x94 wherein n is 1 to 12, diethylene cyclohexane, 1,2,4-triethylene cyclohexane, and diethylene benzene. It is preferred that the sum of the carbon atoms within the G groups within the molecule is from 3 to 18, more preferably 6 to 14. This amount of carbon in the blocked mercaptosilane facilitates the dispersion of the inorganic filler into the organic polymers, thereby improving the balance of properties in the cured, filled rubber.
Specific examples of X are methoxy, ethoxy, isobutoxy, propoxy, isopropoxy, acetoxy oximato, methyl, and phenyl. Methoxy, acetoxy and ethoxy are preferred. At least one X must be reactive (i.e., hydrolyzable).
Preferably, p is 0 to 2; X is ROxe2x80x94 or RC(xe2x95x90O)Oxe2x80x94; R is hydrogen, methyl, ethyl, isopropyl, or isobutyl; G is a substituted phenyl or a substituted straight chain C2 to C12 alkyl. More preferably, p is zero, X is ethoxy, and G is a C3-C12 alkyl derivative.
The latent mercaptosilanes of the present invention are intended also to include partial hydrolyzates of the compounds having the general structure depcited by Formulae 1 and 2, in that these partial hydrolyzates will be a side product of most methods of manufacture of the latent mercaptosilanes or can occur upon storage of the aforementioned, especially in humid conditions.
As used herein, xe2x80x9calkylxe2x80x9d includes straight, branched, and cyclic alkyl groups; xe2x80x9calkenylxe2x80x9d includes straight, branched, and cyclic alkenyl groups containing one or more carbonxe2x80x94carbon double bonds; and xe2x80x9calkynylxe2x80x9d includes straight, branched, and cyclic alkynyl groups containing one or more carbonxe2x80x94carbon triple bonds and, optionally, one or more carbonxe2x80x94carbon double bonds. Specific alkyls include methyl, ethyl, propyl, and isobutyl. Specific examples of alkenyls include vinyl propenyl, allyl, methallyl, ethylidenyl norbornane, ethylidene norbornyl, ethylidenyl norbornene and ethylidene norbornenyl. Specific examples of alkynyls include acetylenyl, propargyl, and methylacetylenyl. As used herein, xe2x80x9carylxe2x80x9d includes any aromatic hydrocarbon from which one hydrogen atom has been removed; xe2x80x9caralkylxe2x80x9d includes any alkyl groups in which one or more hydrogen atoms have been substituted with aryl substituents; and xe2x80x9carenylxe2x80x9d includes any aryl groups in which one or more hydrogen atoms have been substituted by alkyl substituents. Specific examples of aryls include phenyl and naphthalenyl. Specific examples of aralkyls include benzyl and phenethyl. Specific examples of arenyl include tolyl and xylyl. As used herein, xe2x80x9ccyclic alkyl,xe2x80x9d xe2x80x9ccyclic alkenyl,xe2x80x9d and xe2x80x9ccyclic alkynylxe2x80x9d also include bicyclic, tricyclic, and higher cyclic structures, as well as the aforementioned cyclic structures further substituted with alkyl, alkenyl, and/or alkynyl groups. Representive examples include norbomyl, norbomenyl, ethylnorbomyl, ethylnorbomenyl, ethylcyclohexyl, ethylcyclohexenyl, cyclohexylcyclohexyl, and cyclododecatrienyl.
Representative examples of the latent mercaptosilanes useful in the present invention include 2-triethoxysilyl-1-ethyl thioacetate; 2-trimethoxysilyl-1-ethyl thioacetate; 2-(methyldimethoxysilyl)-1-ethyl thioacetate; 3-trimethoxysilyl-1-propyl thioacetate; triethoxysilylmethyl thioacetate; trimethoxysilylmethyl thioacetate; triisopropoxysilylmethyl thioacetate; methyldiethoxysilylmethyl thioacetate; methyldimethoxysilylmethyl thioacetate; methyldiisopropoxysilylmethyl thioacetate; dimethylethoxysilylmethyl thioacetate; dimethylmethoxysilylmethyl thioacetate; dimethylisopropoxysilylmethyl thioacetate; 2-triisopropoxysilyl-1-ethyl thioacetate; 2-(methyldiethoxysilyl)-1-ethyl thioacetate; 2-(methyldiisopropoxysilyl)-1-ethyl thioacetate; 2-(dimethylethoxysilyl)-1-ethyl thiioacetate; 2-(dimethylmethoxysilyl)-1-ethyl thioacetate; 2-(dimethylisopropoxysilyl)-1-ethyl thioacetate; 3-triethoxysilyl-1-propyl thioacetate; 3-triisopropoxysilyl-1-propyl thioacetate; 3-methyldiethoxysilyl-1-propyl thioacetate; 3-methyldimethoxysilyl-1-propyl thioacetate; 3-methyldiisopropoxysilyl-1-propyl thioacetate; 1-(2-triethoxysilyl-1-ethyl)-4-thioacetylcyclohexane; 1-(2-triethoxysilyl-1-ethyl)-3-thioacetylcyclohexane; 2-triethoxysilyl-5-thioacetylnorbornene; 2-triethoxysilyl-4-thioacetylnorbomene; 2-(2-triethoxysilyl-1-ethyl)-5-thioacetylnorbornene; 2-(2-triethoxysilyl-1-ethyl)-4-thioacetylnorbornene; 1-(1-oxo-2-thia-5-triethoxysilylpenyl)benzoic acid; 6-triethoxysilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-hexyl thioacetate; 8-triethoxysilyl-1-octyl thioacetate; 1-triethoxysilyl-7-octyl thioacetate; 6-triethoxsilyl-1-hexyl thioacetate; 1-triethoxysilyl-5-octyl thioacetate; 8-trimethoxysilyl-1-octyl thioacetate; 1-trimethoxysilyl-7-octyl thioacetate; 10-triethoxysilyl-1-decyl thioacetate; 1-triethoxysilyl-9-decyl thioacetate; 1-triethoxysilyl-2-butyl thioacetate; 1-triethoxysilyl-3-butyl thioacetate; 1-triethoxysilyl-3-methyl-2-butyl thioacetate; 1-triethoxysilyl-3-methyl-3-butyl thioacetate; 1-trimethoxysilyl-1-propyl thiooctoate; 3-triethoxysilyl-1-propyl thiopalmitate; 3-triethoxysilyl-1-propyl thiooctoate; 3-triethoxysilyl-1-propyl thiobenzoate; 3-triethoxysilyl-1-propyl thio-2-ethylhexanoate; 3-methyldiacetoxysilyl-1-propyl thioacetate; 3-triacetoxysilyl-1-propyl thioacetate; 2-methyldiacetoxysilyl-1-ethyl thioacetate; 2-triacetoxysilyl-1-ethyl thioacetate; 1-methyldiacetoxysilyl-1-ethyl thioacetate; 1-triacetoxysilyl-1-ethyl thioacetate; tris-(3-triethoxysilyl-1-propyl)trithiophosphate; bis-(3-triethoxysilyl-1-propyl)methyldithiophosphonate; bis-(3-triethoxysilyl-1-propyl)ethyldithiophosphonate; 3-triethoxysilyl-1-propyldimethylthiophosphinate; 3-triethoxysilyl-1-propyldiethylthiophosphinate; tris-(3-triethoxysilyl-1-propyl)tetrathiophosphate; bis-(3-triethoxysilyl-1-propyl)methyltrithiophosphonate; bis-(3-triethoxysilyl-1-propyl)ethyltrithiophosphonate; 3-triethoxysilyl-1-propyldimethyldithiophosphinate; 3-triethoxysilyl-1-propyldiethyldithiophosphinate; tris-(3-methyldimethoxysilyl-1-propyl)trithiophosphate; bis-(3-methyldimethoxysilyl-1-propyl)methyldiithiophosphonate; bis-(3-methyldimethoxysilyl-1-propyl)ethyldithiodophosphonate; 3-methyldimethoxysilyl-1-propyldimethylthiophosphinate; 3-methyldimethoxysilyl-1-propyldiethylthiophosphinate; 3-triethoxysilyl-1-propylmethylthiosulphate; 3-triethoxysilyl-1-propylmethanethiosulphonate; 3-triethoxysilyl-1-propylethanethiosulphonate; 3-triethoxysilyl-1-propylbenzenethiosulphonate; 3-triethoxysilyl-1-propyltoluenethiosulphonate; 3-triethoxysilyl-1-propylnaphthalenethiosulphonate; 3-triethoxysilyl-1-propylxylenethiosulphonate; triethoxysilylmethylmethylthiosulphate; triethoxysilylmethylmethanethiosulphonate; triethoxysilylmethylethanethiosulphonate; triethoxysilylmethylbenzenethiosulphonate; triethoxysilylmethyltoluenethiosulphonate; triethoxysilylmethylnaphthalenethiosulphonate; and triethoxysilylmethylxylenethiosulphonate.
Where an additional mercaptan is employed in the practice of the present invention, it preferably comprises at least one compound having the general structure depicted by Formula 4:
(HSxe2x80x94)rG(xe2x80x94SiX3)txe2x80x83xe2x80x83(4) 
wherein G, X, r, and t are as defined above. When t is 1 to 5, the compounds are mercaptosilanes, which are preferred.
Representative examples of useful additional mercaptans include those wherein G and X are the same as those included in the above description of the latent mercaptosilanes. Thus, representative examples of G include xe2x80x94(CH2)nxe2x80x94 wherein n is 1 to 12; diethylene cyclohexane; 1,2,4-triethylene cyclohexane; and diethylene benzene. G is preferably xe2x80x94CH2xe2x80x94, xe2x80x94(CH2)2xe2x80x94, or xe2x80x94(CH2)3xe2x80x94; more preferably, xe2x80x94(CH2)3xe2x80x94.
Specific examples of X in the additional mercaptans useful herein are methoxy, ethoxy, isobutoxy, propoxy, isopropoxy, acetoxy oximato, methyl, and phenyl. Methoxy, acetoxy, and ethoxy are preferred. At least one X must be reactive (i.e., hydrolyzable). In a highly preferred embodiment, three ethoxy groups are present.
Representative examples of the additional mercaptans useful in the present invention, when they are the preferred mercaptosilanes, include 3-mercapto-1-propyltriethoxysilane, 2-mercapto-1-ethyltriethoxysilane, mercaptomethyltriethoxysilane, 8-mercapto-1-octyltriethoxysilane, 10-mercapto-1-decyltriethoxysilane, 1-mercapto-1-ethyltriethoxysilane, 3-mercapto-1-propylmethyldiethoxysilane, 3-mercapto-1-propyldimethylethoxysilane, 3-mercapto-1-propyltrimethoxysilane, 2-mercapto-1-ethyltrimethoxysilane, mercaptomethyltrimethoxysilane, 8-mercapto-1-octyltrimethoxysilane, 10-mercapto-1-decyltrimethoxysilane, 1-mercapto-1 ethyltrimethoxysilane, 3-mercapto-1-propylmethyldimethoxysilane, 3-mercapto-1-propyldimethylmethoxysilane, 3-mercapto-1-propyltripropoxysilane, 3-mercapto-1-propyltriisopropoxysilane, and 3-mercapto-1-propyltributoxysilane.
Mixtures of various additional mercaptans and/or latent mercaptosilanes may be used, for example, where synthetic methods result in a distribution of various silanes or where mixtures of mercaptans and/or latent mercaptosilanes are used for their various blocking or leaving functionalities. Moreover, it should be understood that the partial hydrolyzates of these additional mercaptans and/or latent mercaptosilanes (i.e., mercaptosiloxanes and/or latent mercaptosiloxanes) may also be encompassed by the additional mercaptans and latent mercaptosilanes of the present invention, in that these partial hydrolyzates will be a side product of most methods of manufacture of these mercaptans and latent mercaptosilanes, or may be generated during their storage, especially under humid conditions.
Where an additional mercaptan is employed in the practice of the present invention, the mixtures can contain from about 50 to about 99.9 weight percent of at least one latent mercaptosilane and from about 0.1 to about 50 weight percent the additional mercaptan, preferably from about 80 to 99 weight percent latent mercaptosilane and from about 1 to 20 weight percent additional mercaptan, and, more preferably, from about 90 to about 98 weight percent latent mercaptosilane and from about 2 to about 10 weight percent additional mercaptan.
The latent mercaptosilane, if liquid, may be loaded on a carrier, such as a porous polymer, carbon black or mineral filler so that it is in solid form for delivery to the rubber. In a preferred mode, the carrier would be part of the filler to be used in the rubber.
The latent-mercaptosilane activators of the present invention comprise at least one compound having the general structure depicted by Formulae 5, 6, and 7:
{U(xe2x95x90Z)qxe2x80x94Zxe2x80x94}cxe2x80x83xe2x80x83(5) 
Rxe2x80x94Zxe2x80x94{U(xe2x95x90Z)qxe2x80x94Zxe2x80x94}mRxe2x80x83xe2x80x83(6) 
{Rxe2x80x94Zxe2x80x94{U(xe2x95x90Z)qxe2x80x94Zdxe2x80x94}g}cLxe2x80x83xe2x80x83(7) 
wherein:
each Z is independently selected from the group consisting of an oxygen atom and NR, wherein R is as defined above;
each R is defined as above;
each U is independently a fragment, obtained by substitution of a quantity of hydrogen atoms given by 2q+2, of any hydrocarbon or oxa-, aza-, and/or cyano-substituted hydrocarbon, U1, having from 1 to 30 carbon atoms. U1 includes, but is not limited to, branched, straight-chain, cyclic, and/or polycyclic aliphatic hydrocarbons, optionally containing ether functionality via oxygen atoms, each of which is bound to two separate carbon atoms; tertiary amine functionality via nitrogen atoms, each of which is bound to three separate carbon atoms, and/or cyano (CN) groups; aromatic hydrocarbons; and arenes derived by substitution of the aforementioned aromatics with branched or straight chain alkyl, alkenyl, alkynyl, aryl, and/or aralkyl groups;
each L is independently a fragment, obtained by substitution of a quantity of hydrogen atoms given by c, of ammonia and any hydrocarbon or oxa-, aza-, and/or cyano-substituted hydrocarbon, L1, having from 1 to 30 carbon atoms. L1 includes, but is not limited to, branched, straight-chain, cyclic, and/or polycyclic aliphatic hydrocarbons, optionally containing ether functionality via oxygen atoms, each of which is bound to two separate carbon atoms; tertiary amine functionality via nitrogen atoms, each of which is bound to three separate carbon atoms, and/or cyano (CN) groups; aromatic hydrocarbons; and arenes derived by substitution of the aforementioned aromatics with branched or straight chain alkyl, alkenyl, alkynyl, aryl and/or aralkyl groups;
m is an integer of from 0 to 500; g is 0 to 1; d is 0 to 2, with the proviso that d is not 2 if Z is oxygen; and c and q are defined as above.
Representative examples of R in the latent-mercaptosilane activators (structures given by Formulae 5, 6, and 7) include phenyl, as for example in any of the isomers of diphenylguanidine (Formula 6); benzyl; cyclohexyl; methyl; ethyl; propyl; iso-propyl; butyl; sec-butyl; tert-butyl; and neopentyl.
Representative examples of U in these same structures include carbon, as, for example, in any of the isomers of diphenylguanidine (Formula 6); xe2x80x94CH2xe2x80x94, as, for example, in glycerol (Formula 7); xe2x80x94CH2CH2xe2x80x94, as, for example, in the cyclic (Formula 5) or linear (Formula 6) ethyleneamines, as in the polyethylene or mixed polyethylene-polypropylene glycols (Formula 6), and as in ethanolamine or diethanolamine (Formula 6); and xe2x80x94CH2CH(CH3)xe2x80x94, as, for example, in the polypropylene or xe2x80x94CH2CH2xe2x80x94 and xe2x80x94CH2CH(CH3)xe2x80x94, as for example, mixed polyethylene-polypropylene glycols (Formula 6).
Representative examples of L in these structures (Formula 7) include CH3C(xe2x88x92)3, as, for example, in trimethylolethane; C2H5C(xe2x88x92)3, as, for example, in trimethylolpropane; CH(xe2x88x92)3, as, for example, in glycerol; and N(xe2x88x92)3, as, for example, in triethanolamine.
Representative examples of latent-mercaptosilane activators useful in the present invention include N,Nxe2x80x2-diphenylguanidine (DPG) and any of its isomers; the ethanolamines, ethanolamine, diethanolamine, and triethanolamine; the ethyleneamines, such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, etc.; ethylene glycol and the polyethylene glycols, such as diethylene glycol, triethylene glycol, tetraethylene glycol, etc.; propylene glycol and the polypropylene glycols, such as dipropylene glycol, tripropylene glycol, tetrapropylene glycol, etc.; any of the mixed ethylene-propylene glycols; any of the aforementioned glycols terminated on one end by an alkyl group, such as 2-methoxyethanol, 2-ethoxyethanol, 2-butoxyethanol, 1-methoxyisopropanol, 1-ethoxyisopropanol, and 1-butoxyisopropanol, 2-(2-methoxyethoxy)ethanol, 2-(2-ethoxyethoxy)ethanol, 2-(2-butoxyethoxy)ethanol, and 2-(2-hexoxyethoxy)ethanol; glycerol; any of the trimethylolalkanes, such as trimethylolethane and trimethylolpropane; pentaerythritol; any of the anilines and phenylene diamines; phenol; catechol; dihydroquinone; resorcinol; any of the aminophenols; 1,3-diaminopropane; 1,4-diaminobutane; 1,5-diaminopentane; 1,6-diaminohexane; N-(3-aminopropyl)-1,3-propanediamine (3,3xe2x80x2-iminobispropylamine); 3-amino-1-propanol; imidazole; benzimidazole; any of the isomers of aminobenzimidazole; pyrrole; indole; pyrazole; triazole; and benzotriazole.
In a preferred embodiment of the present invention, latent-mercaptosilane activators are used that have, as a minimum, a sufficiently low volatility that losses due to evaporation are minimized at the temperatures used during processing of the rubber, which can be up to about 200xc2x0 C. Preferred latent-mercaptosilane activators are also capable of functioning efficiently as a proton source and blocking group acceptor so that they can de-block and thereby activate the latent mercaptosilane by the in situ formation of a mercaptosilane from the latent mercaptosilane.
Highly preferred embodiments of the present invention include the use of latent-mercaptosilane activators selected from the group consisting of N,Nxe2x80x2-diphenylguanidine (DPG); glycerol; N-(3-aminopropyl)-1,3-propanediamine (3,3xe2x80x2-iminobispropylamine); diethylene triamine; triethylene tetramine; trimethylolpropane; and ethylene glycol. The most preferred latent-mercaptosilane activators are DPG, glycerol, diethylene glycol, and trimethylol propane.
The elastomers useful with the coupling agents described herein include sulfur vulcanizable rubbers including conjugated diene homopolymers and copolymers, and copolymers of at least one conjugated diene and at least one aromatic vinyl compound. Suitable organic polymers for the preparation of rubber compositions are well known in the art and are described in various textbooks, including The Vanderbilt Rubber Handbook, Ohm, R. F., R. T. Vanderbilt Company, Inc., 1990, and the Manual for the Rubber Industry, Kemperman, T. and Koch, S. Jr., Bayer A G, Leverkusen, 1993.
One example of a suitable polymer for use herein is solution-prepared styrene-butadiene rubber (sSBR). This polymer typically has a bound styrene content in the range of from 5 to 50, preferably from 9 to 36 weight percent and a vinyl content from 10 to 60 weight percent, and preferably 20 to 55 weight percent. Other useful polymers include styrene-butadiene rubber (SBR), natural rubber (NR), ethylene-propylene copolymers and terpolymers (EP, EPDM), acrylonitrile-butadiene rubber (NBR), polybutadiene (BR), and the like.
The rubber composition comprises at least one diene-based elastomer, or rubber. Suitable conjugated dienes are isoprene and 1,3-butadiene and suitable vinyl aromatic compounds are styrene and alpha methyl styrene. Polybutadiene can be characterized as existing primarily (typically about 90% by weight) in the cis-1,4-butadiene form.
Thus, the rubber is a sulfur curable rubber. Such diene based elastomer, or rubber, may be selected, for example, from at least one of cis-1,4-polyisoprene rubber (natural and/or synthetic), emulsion polymerization prepared styrene/butadiene copolymer rubber, organic solution polymerization prepared styrene/butadiene rubber, 3,4-polyisoprene rubber, isoprene/butadiene rubber, styrene/isoprene/butadiene terpolymer rubber, cis-1,4-polybutadiene, medium vinyl polybutadiene rubber (35-50 percent vinyl), high vinyl polybutadiene rubber (50 to 75 percent vinyl), styrene/isoprene copolymers, emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubber and butadiene/acrylonitrile copolymer rubber.
For some applications, an emulsion polymerization derived styrene/butadiene (E-SBR) having a relatively conventional styrene content of about 20 to 28 percent bound styrene, or an E-SBR having a medium to relatively high bound styrene content of about 30 to 45 percent may be used.
Emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubbers containing 2 to 40 weight percent bound acrylonitrile in the terpolymer are also contemplated as diene based rubbers for use in this invention.
A particulate filler is also added to the crosslinkable elastomer compositions of the present invention, including siliceous fillers, other mineral fillers, carbon black, and the like. The filler materials useful herein include, but are not limited to, metal oxides, such as silica (pyrogenic and precipitated), titanium dioxide, aluminosilicate and alumina, clays and talc, silica modified carbon black, carbon black, and the like.
Particulate, precipitated silica is also sometimes used for such purpose, particularly when the silica is used in conjunction with a silane. In some cases, a combination of silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires. Alumina can be used either alone or in combination with silica. The term, alumina, can be described herein as aluminum oxide, or Al2O3. The fillers may be hydrated or in anhydrous form. Use of alumina in rubber compositions is described, for example, in U.S. Pat. No. 5,116,886 and EP 631 982.
The latent mercaptosilane(s) can be premixed or pre-reacted with the filler particles, or can be added to the rubber mix during the rubber and filler processing, or mixing stages. If the latent mercaptosilanes and filler are added separately to the rubber mix during the rubber and filler mixing, or processing stage, it is considered that the latent mercaptosilane(s) then combine(s) in an in situ fashion with the filler.
The vulcanized rubber composition should contain a sufficient amount of filler to contribute a reasonably high modulus and high resistance to tear. The combined weight of the filler may be as low as about 5 to about 100 phr but is more preferably from about 25 to about 85 phr.
Preferably, at least one precipitated silica is utilized as a filler. The silica may be characterized by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600 m2/g, more preferably in the range of from about 50 to about 300 m2/g. The BET method of measuring surface area is described in the Journal of the American Chemical Society, 60:304 (1930). The silica typically has a dibutylphthalate (DBP) absorption value in a range of 100 to 350 ml/100 grams, more usually, 150 to 300 ml/100 grams. Further, the silica, as well as the alumina and aluminosilicate mentioned above, may be expected to have a CTAB surface area in a range of 100 to 220 m2/g. The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849.
The average mercury porosity specific surface area for the silica should be in a range of from about 100 to about 300 m2/g. Mercury porosity surface area is the specific surface area determined by mercury porosimetry. Using this method, mercury is penetrated into the pores of the sample after a thermal treatment to remove volatiles. Set up conditions may be suitably described as using a 100 mg sample; removing volatiles over a period of two hours at 105xc2x0 C. and ambient atmospheric pressure; ambient to 2000 bars pressure measuring range. Such an evaluation may be performed according to the method described in Winslow, Shapiro in ASTM bulletin, page 39 (1959) or according to DIN 66133. For such an evaluation, a CARLO-ERBA Porosimeter 2000 might be used.
A suitable pore size distribution for the silica, alumina and aluminosilicate according to such mercury porosity evaluation is considered herein to be such that five is percent or less of its pores have a diameter of less than about 10 nm, 60 to 90 percent of its pores have a diameter of 10 to 100 nm, 10 to 30 percent of its pores have a diameter at 100 to 1,000 nm, and 5 to 20 percent of its pores have a diameter of greater than about 1,000 nm.
The silica typically has an average ultimate particle size in the range of, for example, 10 to 50 nm as determined by the electron microscope, although the silica particles may be even smaller or, possibly, larger in size. Various commercially available silicas may be considered for use in this invention, such as HI-SIL 210, 243, etc. (PPG Industries); ZEOSIL 1165MP (Rhodia); ULTRASIL VN2, VN3, and 7000GR, etc. (Degussa); and ZEOPOL 8745 and 8715 (Huber).
In compositions for which it is desirable to utilize siliceous fillers, such as silica, alumina, and/or aluminosilicates in combination with carbon black reinforcing pigments, the compositions may comprise a filler mix of from about 15 to about 95 weight percent of the siliceous filler, and from about 5 to about 85 weight percent carbon black, wherein the carbon black has a CTAB value in a range of 80 to 150 m2/g. More typically, it is desirable to use a weight ratio of siliceous fillers to carbon black of at least about 1/1, and preferably at least about 3/1. The siliceous filler and carbon black may be pre-blended or added separately during mixing of the vulcanizable rubber.
In preparing the rubber compositions of the present invention, at least one of the latent mercaptosilanes is mixed with the organic polymer before, during, or after the compounding of the filler into the organic polymer. Addition of the latent mercaptosilanes during the compounding of the filler into the organic polymer is preferred, because these silanes facilitate and improve the dispersion of the filler. The total amount of latent mercaptosilane present in the resulting combination should be about 0.05 to about 25 parts by weight per hundred parts by weight of filler; more preferably 1 to 10 parts.
In practice, sulfur vulcanized rubber products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequential, step-wise, manner, followed by shaping and heating the compounded rubber to form a vulcanized (cured) product.
First, for the mixing of the rubber and various ingredients, usually exclusive of sulfur and sulfur vulcanization accelerators (collectively, curing agents), the rubber(s) and various rubber compounding ingredients typically are blended in at least one, and often (in the case of silica filled low rolling resistance tires) two or more, preparatory thermomechanical mixing stage(s) in suitable mixers. Such preparatory mixing is referred to as non-productive mixing or nonproductive mixing steps or stages. Such preparatory mixing usually is conducted at temperatures of about 140xc2x0 C. to 200xc2x0 C., usually about 150xc2x0 C. to 180xc2x0 C., in the mixer.
Subsequent to such preparatory mix stages, in a final mixing stage, sometimes referred to as a productive mix stage, curing agents, and possibly one or more additional ingredients, are mixed with the rubber compound or composition, at lower temperatures of typically about 50xc2x0 C. to about 110xc2x0 C. in order to prevent or retard premature curing of the sulfur curable rubber, sometimes referred to as scorching. The rubber mixture, also referred to as a rubber compound or composition, typically is allowed to cool, for example, to a temperature of about 50xc2x0 C. or lower, sometimes after or during a process intermediate mill mixing, between the various mixing steps. When it is desired to mold and to cure the rubber, it is formed into an appropriate shape and brought to a temperature of at least about 130xc2x0 C., and up to about 200xc2x0 C., which will cause the vulcanization of the rubber by the mercapto groups on the mercaptosilane and the other sulfur sources in the rubber mixture.
Thermomechanical mixing refers to the phenomenon whereby, owing to the shear forces and associated friction occurring as a result of mixing the rubber compound, or some blend of the rubber compound itself and rubber compounding ingredients, in a high shear mixer, the temperature autogeneously increases, i.e., it xe2x80x9cheats up.xe2x80x9d
Several chemical reactions may occur at various steps in the mixing and curing processes.
The first reaction is a relatively fast reaction and is considered herein to take place between the filler and the xe2x80x94SiX3 group of the latent mercaptosilane. Such a reaction may occur at a relatively low temperature, such as, for example, about 120xc2x0 C. The second reaction is considered herein to be the reaction that takes place between the sulfur-containing portion of the latent mercaptosilane and the sulfur vulcanizable rubber at a higher temperature, for example, above about 140xc2x0 C.
Another sulfur source may be used, for example, in the form of elemental sulfur, such as, but not limited to, S8. A sulfur donor is considered herein as a sulfur-containing compound that liberates free, or elemental, sulfur at a temperature in a range of from about 140xc2x0 C. to about 190xc2x0 C. Such sulfur donors include, but are not limited to, polysulfide vulcanization accelerators and organosilane polysulfides with at least three connecting sulfur atoms in the polysulfide bridge. The amount of free sulfur source addition to the mixture can be controlled or manipulated as a matter of choice relatively independent of the addition of the latent mercaptosilane. Thus, for example, the independent addition of a sulfur source may be manipulated by the amount of addition thereof and by the sequence of addition relative to the addition of other ingredients to the rubber mixture.
A desirable rubber composition may therefore comprise:
(1) about 100 parts by weight of at least one sulfur vulcanizable rubber selected from the group consisting of conjugated diene homopolymers and copolymers and copolymers of at least one conjugated diene and at least one aromatic vinyl compound,
(2) about 5 to 100 parts, preferably about 25 to 80 parts, per 100 parts by weight rubber of at least one particulate filler,
(3) up to about 5 parts by weight per 100 parts by weight rubber of a curing agent,
(4) from 0.5 to 10 parts by weight per 100 parts by weight rubber of a latent-mercaptosilane activator,
(5) up to about 10 parts by weight per 100 parts by weight rubber of an additional mercaptan, and
(6) about 0.05 to about 25 parts per hundred parts by weight rubber of at least one latent mercaptosilane, all as described herein.
The filler preferably is from 15 to 100 weight percent siliceous filler, such as silica and from about 0 to about 85 weight percent carbon black based on the total weight of the filler.
The rubber composition is preferably prepared by first blending:
(1) rubber, filler, latent mercaptosilane activator, and latent mercaptosilane, or (Alternative 1) rubber, filler pretreated with all or a portion of the latent mercaptosilane, any remaining latent mercaptosilane, latent mercaptosilane activator, and
(2) any other optional ingredients necessary to provide a useful compound in a thermomechanical mixing step; and
then bringing the temperature to about 140-200xc2x0 C. over a period of about 1 to 20 minutes, preferably about 150-180xc2x0 C. over a period of about 10 to 24 minutes.
The rubber composition can, alternatively, be prepared by adding the latent mercaptosilane activator to the productive mix, rather than during the nonproductive mix.
Where a curing agent is employed, it is added in another thermomechanical productive mixing step at a temperature of from about 25xc2x0 C. to about 110xc2x0 C., more preferably from about 50xc2x0 C. to about 110xc2x0 C., and mixed for about 1 to 30 minutes. After shaping, the temperature is raised again to between about 130xc2x0 C. and about 200xc2x0 C. and curing is accomplished in about 5 to about 60 minutes.
The process may also comprise the additional steps of preparing an assembly of a tire or sulfur vulcanizable rubber with a tread comprised of the rubber composition prepared according to the present invention and vulcanizing the assembly at a temperature in a range of from about 130xc2x0 C. to about 200xc2x0 C.
Optional ingredients that may be added to the rubber compositions of the present invention include curing aids, i.e. sulfur compounds, including activators, retarders and accelerators, processing additives, such as oils, plasticizers, tackifying resins, silicas, other fillers, pigments, fatty acids, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents, reinforcing materials such as, for example, carbon black, and the like. Any such additives are selected based upon the intended use and on the sulfur vulcanizable material selected for use, which selections are within the knowledge of those skilled in the art, as are the required amounts of such additives.
The vulcanization may be conducted in the presence of additional sulfur vulcanizing agents. Examples of suitable sulfur vulcanizing agents include, for example, elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amino disulfide, polymeric polysulfide or sulfur olefin adducts that are conventionally added in the final, productive, rubber composition mixing step. The sulfur vulcanizing agents (which are common in the art) are used, or added in the productive mixing stage, in an amount ranging from about 0.4 to about 3 phr, or even, in some circumstances, up to about 8 phr, with a range of from about 1.5 to about 2.5 phr being preferred.
Optionally, vulcanization accelerators may be used herein. It is appreciated that they may be, for example, of the type such as, for example, benzothiazole, alkyl thiuram disulfide, guanidine derivatives, and thiocarbamates. Examples of such accelerators include, but not limited to, mercapto benzothiazole, tetramethyl thiuram disulfide, benzothiazole disulfide, diphenylguanidine, zinc dithiocarbamate, alkylphenoldisulfide, zinc butyl xanthate, N-dicyclohexyl-2-benzothiazolesulfenamide, N-cyclohexyl-2-benzothiazolesulfenamide, N-oxydiethylenebenzothiazole-2-sulfenamide, N,N-diphenylthiourea, dithiocarbamylsulfenamide, N,N-diisopropylbenzothiozole-2-sulfenamide, zinc-2-mercaptotoluimidazole, dithiobis(N-methyl piperazine), dithiobis(N-beta-hydroxy ethyl piperazine) and dithiobis(dibenzyl amine).
Additionally, sulfur donors may be used, for example, thiuram and morpholine derivatives. Examples of such donors include, but are not limited to, dimorpholine disulfide, dimorpholine tetrasulfide, tetramethyl thiuram tetrasulfide, benzothiazyl-2,N-dithiomorpholide, thioplasts, dipentamethylenethiuram hexasulfide, and disulfidecaprolactam.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., a primary accelerator. Conventionally and preferably, at least one primary accelerator is used in a total amount ranging from about 0.5 to about 4, preferably about 0.8 to about 1.5 phr. Combinations of a primary and a secondary accelerator may be used, with the secondary accelerator being used in smaller amounts (about 0.05 to about 3 phr) in order to activate and improve the properties of the vulcanizate. Suitable types of accelerators include amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate, or thiuram compound. Delayed action accelerators may be used. Vulcanization retarders might also be used.
Tackifier resins, if used, are typically employed at a level of from about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids are from about 1 to about 50 phr. Such processing aids can include, for example, aromatic, naphthenic, and/or paraffinic processing oils. Typical amounts of antioxidants are from about 1 to about 5 phr. Representative antioxidants include diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in the Vanderbilt Rubber Handbook (1978), pages 344 to 346. Typical amounts of antiozonants are from about 1 to about 5 phr. Typical amounts of fatty acids (which can include stearic acid), if used, are from about 0.5 to about 3 phr. Typical amounts of zinc oxide are about 2 to about 5 phr. Typical amounts of waxes are from about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers are from about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
The rubber composition of this invention can be used for various purposes. For example, it can be used for various tire compounds. Such tires can be built, shaped, molded and cured by various methods which are known and will be readily apparent to those having skill in such art.
In the practice of the present invention, the use of the latent-mercaptosilane activators and/or the mercaptosilanes leads to improvements in the utilization of the latent mercaptosilanes as coupling agents for mineral-filled elastomers. The combination of (1) the use of higher mixing temperatures than those typically used in the art, (2) the introduction of activators, such as N,Nxe2x80x2-diphenylguanidine (DPG), during the non-productive mixing stage, (3) the use of the latent mercaptosilanes as coupling agents, and (4) optionally, the use of smaller amounts of mercaptans, preferably mercaptosilanes permits the elimination of an entire processing step in the processing of mineral-filled elastomers. This elimination of a processing step is highly advantageous in that it leads to substantial cost reductions in the compounding of mineral-filled elastomers. The use of the latent mercaptosilane leads to significant reductions in compound viscosity compared to the viscosities obtained by current technology. It also permits the use of higher mixer fill factors, resulting in substantial gains in production throughput with existing capacity. Adjustments in the way the filler is added during mixing results in better filler dispersion, compound hysteresis, and reinforcement properties.
Various features and aspects of the present invention are illustrated further in the examples that follow. While these examples are presented to show one skilled in the art how to operate within the scope of the invention, they are not intended in any way to serve as a limitation upon the scope of the invention.